1. Field of the Invention
The present invention relates to an adjustment method, an exposure method, a device manufacturing method, and an exposure apparatus.
2. Description of the Related Art
Conventionally, a projection exposure apparatus configured to project a circuit pattern formed on a reticle (mask) onto a wafer using a projection optical system to transfer the circuit pattern is used in manufacturing a semiconductor device employing photolithography techniques.
There are three important factors in determining the exposure performance of a projection exposure apparatus: resolution, overlay accuracy, and throughput. In recent years, resolution is especially attracting attention in relation with the development of increased numerical aperture (NA), which has become feasible with the use of an immersion projection optical system. Increased NA in a projection optical system leads to an increased angle between the perpendicular to an image plane and the traveling direction of incident light. This is referred to as high NA imaging.
In high NA imaging, the polarization state of exposure light becomes important. For example, light that has a polarization direction parallel to a repetitive pattern (e.g., repetitive line-and-space pattern) is referred to as s-polarized light, and light that has a polarization direction perpendicular to the repetitive pattern is referred to as p-polarized light. In the case where the angle defined by interference light beams is 90 degrees, since interference occurs with s-polarized light, s-polarized light can form a light intensity distribution corresponding to the pattern on an image plane. On the other hand, since interference does not occur with p-polarized light, p-polarized light forms a uniform light intensity distribution and, thus, cannot form a light intensity distribution corresponding to the pattern on the image plane. If exposure light contains both s-polarized light and p-polarized light, then a light intensity distribution with inferior contrast is formed compared to when exposure light contains only s-polarized light. If the percentage of p-polarized light increases, then the contrast of a light intensity distribution on the image plane decreases, thus resulting in no transfer of the pattern.
Accordingly, it is necessary to improve the contrast by controlling the polarization state of exposure light. Controlling polarization of exposure light enables a light intensity distribution with sufficient contrast to be formed on an image plane. Accordingly, a finer pattern can be formed on the image plane.
Polarization of exposure light can be controlled by controlling the polarization state on a pupil plane in an illumination optical system. However, even if the polarization state on a pupil plane in the illumination optical system is controlled, the controlled polarization state is not always maintained on the image plane. This is because an optical system subsequent to the pupil plane in the illumination optical system or a projection optical system exerts an influence on the polarization state.
For example, in order to improve transmittance or reflectance, an antireflection film may be formed on a lens or a high-reflectance film may be formed on a reflection mirror. However, the transmittance and reflectance of these films depend on a polarization direction. When a phase difference is applied to the films, the polarization state on the image plane may be changed. Further, according to the use of a light source having shorter wavelength as exposure light, quartz or fluorite may be used as a lens material. Such a member causes a change in polarization state due to its birefringence. Further, according to stress caused by a mechanical member, such as a lens barrel, that holds the lens material, birefringence of the lens material may be changed.
Accordingly, a polarization state in an exposure apparatus as a whole needs to be measured. Japanese Patent Application Laid-Open No. 2007-59566 discusses an exposure apparatus that is capable of measuring the polarization state of an illumination optical system or a projection optical system.
However, the exposure apparatus discussed in Japanese Patent Application Laid-Open No. 2007-59566 does not measure the polarization state of an illumination optical system or a projection optical system in a state where a reticle is set on a reticle stage. In other words, the exposure apparatus does not measure the polarization state in a state where the reticle is located between the illumination optical system and the projection optical system. Since actual exposure is performed with the reticle set between the illumination optical system and the projection optical system, the polarization state may be changed due to the birefringence of the reticle itself or to the stress birefringence caused when the reticle is held. Accordingly, although the exposure apparatus discussed in Japanese Patent Application Laid-Open No. 2007-59566 measures the polarization state of the illumination optical system or the projection optical system, the measured polarization state is not the polarization state under the actual exposure conditions.